Exclusive electroproduction of J/psi mesons at HERA

TL;DR

ZEUS measured exclusive J/ψ electroproduction at HERA across two Q^2 regimes, mapping cross sections as functions of W, Q^2, and t and extracting decay angular distributions. The data support perturbative QCD descriptions, reveal a strong sensitivity to the gluon content of the proton, and show SCHC with a rising R = σ_L/σ_T with Q^2. A Regge-inspired Pomeron trajectory is extracted with parameters consistent with J/ψ photoproduction, and the t-slope remains roughly constant with Q^2, aligning with two-gluon exchange expectations. These measurements provide valuable constraints on gluon PDFs and motivate future NLO QCD analyses to exploit exclusive J/ψ production in global fits.

Abstract

The exclusive electroproduction of J/psi mesons, ep->epJ/psi, has been studied with the ZEUS detector at HERA for virtualities of the exchanged photon in the ranges 0.15<Q^2<0.8 GeV^2 and 2<Q^2<100 GeV^2 using integrated luminosities of 69 pb^-1 and 83 pb^-1, respectively.The photon-proton centre-of-mass energy was in the range 30<W<220 GeV and the squared four-momentum transfer at the proton vertex |t|<1.The cross sections and decay angular distributions are presented as functions of Q^2, W and t. The effective parameters of the Pomeron trajectory are in agreement with those found in J/psi photoproduction. The spin-density matrix elements, calculated from the decay angular distributions, are consistent with the hypothesis of s-channel helicity conservation. The ratio of the longitudinal to transverse cross sections, sigma_L/sigma_T, grows with Q^2, whilst no dependence on W or t is observed. The results are in agreement with perturbative QCD calculations and exhibit a strong sensitivity to the gluon distribution in the proton.

Figures (10)

• Figure 1: The distribution of the events in the muon and electron channels in the kinematic plane of Bjorken-x and $Q^2$. The events reconstructed using one and two measured tracks are shown separately.
• Figure 2: Invariant mass distributions of the lepton pairs for (a) the low-$Q^2$ sample and (b)-(e) the high-$Q^2$ sample. The shaded histograms are the QED MC distributions and the open histograms the sum of the $J/\psi$ and QED MC events. The small excess of data at low mass is due to background from pions. The error bars indicate the statistical uncertainties.
• Figure 3: Exclusive $J/\psi$ electroproduction cross section (a) as a function of $W$ for four values of $Q^2$ and (b) as a function of $Q^2$ at $\langle W\rangle=90{\,\text{Ge}\text{V}}$. ZEUS photoproduction and H1 electroproduction cross sections are also shown. The full lines are fits to the ZEUS data of the form (a) $\sigma \propto W^{\delta(Q^2)}$ and (b) $\sigma \propto (Q^2+M_{J/\psi}^2)^{-n}$. The inner error bars represent the statistical uncertainties, the outer bars are the statistical and systematic uncertainties added in quadrature. An overall normalisation uncertainty of $^{+5\%}_{-8\%}$ was not included.
• Figure 4: Exclusive $J/\psi$ electroproduction cross section (a) as a function of $W$ for four values of $Q^2$ and (b) as a function of $Q^2$ at $\langle W\rangle=90{\,\text{Ge}\text{V}}$. ZEUS photoproduction results are also shown. The curves represent the predictions of the QCD models MRT, FKS and GLLMN (see text) normalised to the ZEUS photoproduction point at $\langle W\rangle=90{\,\text{Ge}\text{V}}$. The insert shows the parameter $\delta$ as a function of $Q^2$. The inner error bars represent the statistical uncertainties, the outer bars are the statistical and systematic uncertainties added in quadrature. An overall normalisation uncertainty of $^{+5\%}_{-8\%}$ was not included.
• Figure 5: Exclusive $J/\psi$ electroproduction cross section (a) as a function of $W$ for four values of $Q^2$ and (b) as a function of $Q^2$ at $\langle W\rangle=90{\,\text{Ge}\text{V}}$. ZEUS photoproduction results are also shown. The data are compared to the MRT predictions (see text) obtained with different parametrisations of the gluon density and normalised to the ZEUS photoproduction point at $\langle W\rangle=90{\,\text{Ge}\text{V}}$. The insert shows the parameter $\delta$ as a function of $Q^2$. The inner error bars represent the statistical uncertainties, the outer bars are the statistical and systematic uncertainties added in quadrature. An overall normalisation uncertainty of $^{+5\%}_{-8\%}$ was not included.